The present invention relates to a pickup for system for recording information on an optical disc and reproducing the information therefrom, and more particularly to a system related to discs such as a write-once digital video disc (DVD-R) and a writable digital video disc (DVD-RAM).
There has been a widespread use of optical discs where information can be recorded in large quantities. A digital video disc (DVD) has been noted as one of these optical discs. The DVD stores digital data such as moving images and audio information compressed in accordance with the MPEG-2 standard. The recorded data is expanded so as to reproduce the information. A read-only DVD reproducing system is now commercialized, and moreover, DVD-R where information can be recorded once by a user, and DVD-RAM where information can be rewritten a number of times, about a hundred thousand times in practice, are being developed for commercialization.
As a recording and reproducing system for the DVD-R and DVD-RAM, there has been proposed the use of a conventional system similar to the one for recording and reproducing a CD-R as shown in FIG. 9.
Referring to FIG. 9, the conventional recording and reproducing system comprises an optical pickup 1 which emits a laser beam for recording information on a disc 4 and reproducing the information from the disc. The operation of the optical pickup 1 is controlled by an optical pickup control means 2 having a track-following servo system 2a and a focus servo system 2b, and a signal processing means 3 for processing signals to be recorded and signals read out from the disc 4.
Information to be recorded on the disc 4 is fed to a modulator 3a of the signal processing means 3 wherein the information is converted into a signal of a predetermined format. The signal is fed to a laser diode 5 of the optical pickup through a driver 3b. The laser diode 5 accordingly emits a laser beam.
The laser beam is transmitted through a collimator lens 6 and split into three beams by a diffraction grating element 15. The diffraction grating element 15 is an ordinary grating, or a hologram, in which case the construction thereof is simplified. The beams are focused on a track of the disc 4 through a polarized beam splitter 7, quarter-wave plate 8 and an objective 9, thereby forming a plurality of pits in grooves of the tracks wherein the reflectance thereof differs from those of the surrounding areas. Hence information is recorded.
When reading out the recorded information, the three beams are applied to a track of the disc 4 and reflected therefrom. The beams are further reflected from the polarized beam splitter 7 and applied to detecting elements of a photodetector device 11, passing through a condenser lens 10. The photodetector device 11 generates a signal in accordance with the detected beams and feeds it to a detected signal processing circuit (not shown) provided in the signal processing means 3 where calculation is made based on the detected signal to produce an RF signal representing the recorded information. The signal is applied to a demodulator 3c where the information signal is demodulated thereby generating a reproduced signal.
The disc 4 has warps and distortions which are inevitably generated during the production thereof. Warps can be further caused by the deadweight of the disc 4 when the disc is mounted on a turntable of the recording and reproducing device. Moreover in some discs, there is a difference between the center of the axis of a spindle for rotating the turntable, and the center of the disc, which causes an eccentricity of the disc. The warp and eccentricity further cause the pickup to fluctuate with respect to the surface of the disc and to deflect from the tracks. The optical pickup control means 2 is provided in order that the laser beam from the optical pickup 1 is correctly focused on each track of the disc 4 for accurate recording and reproduction of the information. Namely, the detected signal processing circuit of the signal processing means 3 generates a tracking error signal TE in accordance with the astigmatic method or other conventional methods, and a focus error signal FE in accordance with the push-pull method. The tracking error signal TE and the focus error signal FE are applied to the track-following servo system 2a and focus servo system 2b, respectively. The track-following servo system 2a and the focus servo system 2b hence operate an actuator 12, thereby appropriately moving the objective 9 to accurately focus the laser beam.
The detected signal processing circuit is described more in detail with reference to FIGS. 10 and 11.
The laser beam emitted from the laser diode 5 is diffracted by the grating element 15 so as to form a center zeroth-order diffraction main beam, and first-order diffraction sub-beams deflected in positive and negative directions from the center zeroth-order diffraction main beam. As shown in FIG. 10, the main beam is projected onto a groove 13 of a track of the disc 4, thereby forming a beam spot 20. The positive and negative sub-beams are projected onto lands 14 formed along the groove 13 on the either side thereof, thereby forming beam spots 21 and 22, respectively. The ratio of the quantities of the light of the zeroth-order beam, positive first-order beam and negative first-order beam is in 20:1:1. The beams reflected from the disc 4 are applied to the photodetector device 11.
Referring to FIG. 11, the photodetector device 11 comprises three photodetectors 30, 31 and 32. The photodetectors 30 and 32 have two detector elements C, D and I, J, respectively, while the photodetector 31 has four detector elements E, F, G, and H. The RF signal RF, tracking error signal TE and the focus error signal FE are obtained based on the outputs of the detector elements C to J. Namely, the RF signal RF is calculated in accordance with EQU RF=(E+F)+(G+H)
The tracking error signal TE is obtained in accordance with EQU TE=[(E+F)-(G+H)]-k[(C+I)-(D+J)]
where k is a coefficient representing an amplification rate, which is, for example 10. The focus error signal FE can be calculated in accordance with EQU FE=(E+H)-(F+G)
Each of the alphabets C to J in the equations represents the quantity of light detected by the detector elements designated by the corresponding alphabet.
When recording information, it is necessary to verify whether the information is accurately recorded. More particularly, after the recording, the recorded information is reproduced as the reproduced signal RF for verifying. A determining means (not shown) is provided for comparing the reproduced information with the original information.
Thus, not only the time for recording but also the time for reading out the recorded information are necessary at the recording operation. For example, in a recording and reproducing system for a DVD-R now being developed, if the information is recorded and reproduced at a linear velocity of 3.7 m/sec without any additional recording, it takes about an hour to record information on the entire recording area formed on the disc. Another hour is required for verification so that a total of two hours is necessary. Moreover, in a disc which already has information recorded in the recording area thereof, the information must be erased before the next recording. Hence longer time is required to erase, record and verify the information. In a magneto-optical (MO) disc, which is now widely used, about three hours is required for the above-described operation.
If another optical pickup is provided exclusively for the verification, the verification time can be reduced, thereby reducing the entire recording time. However, the whole system becomes large in size and heavy in weight, causing the manufacturing cost to rise. Hence the use of the exclusive pickup cannot be adopted.
The optical pickup for a read-only reproducing device for DVDs which is commercialized in some areas of the industrial field, and having the same basic construction as the system described above, reproduces the information one track at a time.